1. Field of the Invention
This invention relates to photosensitive bodies and, in particular, photosensitive bodies that yield positive images.
2. Art Background
Photosensitive bodies are utilized in many lithographic processes such as those involved in semiconductor device fabrication. Generally, the photosensitive body is produced by depositing an appropriately chosen photosensitive material on a substrate, e.g., a silicon wafer. One widely used material involves a physical mixture of a novolak resin such as one formed from cresols and formaldehyde with an insoluble photosensitive inhibitor such as a substituted o-naphthoquinone diazide. The novolak resin is chosen to be soluble in an alkaline solution. The inhibitor is chosen to be insoluble in an alkaline solution in its initial form, but soluble after it has undergone a chemical reaction induced by actinic radiation. In the case of a novolak resin and an o-naphthoquinone diazide inhibitor, upon irradiation with visible light, the inhibitor eliminates nitrogen and forms an indene carboxylic acid. Both the indene carboxylic acid and the resin are base soluble and, therefore, the total mixture is soluble in the areas that are irradiated. The final image is formed by treating these irradiated areas with an alkaline developer.
The previously described photosensitive materials, although quite useful, have some shortcomings. The potential for higher resolution is afforded by the use of short wavelength radiation such as ultraviolet radiation of wavelength less than 300 nm. However, novolak resins are highly absorbing in this portion of the ultraviolet region of the light spectrum. Sufficient incident ultraviolet light for exposure is thus essentially prevented from reaching the lower portion of the material thickness. For this reason, exposure through the thickness of the material for conventional photosensitive bodies having thicknesses greater than 0.5 .mu.m is not practical with ultraviolet radiation of wavelengths less than 300 nm.
Additionally, the contrast of inhibited novolak resins is generally less than 2.5. (For a definition of contrast see UV Curing:Science and Technology, S. P. Pappas, Ed., page 333, Technology Marketing Corp., (1978).) Although this is useful for many applications better contrast is often desirable because it leads to higher image quality. This is especially significant at higher resolution facilitated by the use of short wavelength ultraviolet exposing radiation.
A photosensitive material has been made by converting a base soluble polymer, such as poly(methyl methacrylate-co-methacrylic acid) P(MMA-MAA), that has negligible absorption for radiation of wavelength longer than 230 nm, to a base insoluble polymer by esterification of the carboxylic acid groups with o-nitrobenzyl alcohol to produce a polymer represented by the formula ##STR1## Although no experimental details have been reported, this material is expected to allow absorption of ultraviolet radiation (250 to 300 nm) completely through typical thicknesses. The contrast has not been disclosed. The contrast of any particular organic resist is not predictable and contrast values above 2.5 for organic resists are essentially unknown. There is an absence of photosensitive materials reported to have a high contrast known to be adaptable to ultraviolet (wavelength less than 300 nm) exposure through typical film thicknesses and thus capable of allowing the potential for higher resolution such exposure offers.